Crawling text is text that is shown as overlayed on a program or other content being viewed. The crawling text typically provides secondary information to viewers of a primary program. The secondary information may include weather alerts, advertisements, breaking news, etc. The crawling text is known to cross a designated area of a picture in a predetermined direction, for example, the bottom of a television screen from left to right.
Crawling text may be blended with a video bit stream provided to an end user. In these situations, motion vectors from the original coded video that point into the designated area where the crawling text is inserted, and motion vectors within the text area are often inappropriate and inefficient. The motion vectors are known to be inappropriate and inefficient because the content that they refer to within the crawling text area has changed. For example, when the original motion vectors of a macroblock X are chosen, the area in the reference frame that the original motion vectors pointed into was part of the original unblended video that had a reference area that closely matched macroblock X. After the crawling text is inserted, the original motion vectors point into the same area, but the content has changed and no longer closely matches macroblock X. When the new reference area is used, the differences between macroblock X and the new reference area are much larger than the differences between the original reference area and macroblock X. The number of bits needed to code these larger differences is larger than would be needed if a good match was found and is therefore inefficient. In the area where the text is inserted, the content is changed and so the values of the original macroblock motion vectors are no offer an efficient coding option.